In general, RF power amplifiers are designed according to one of various classes commonly designated as class A, B, C, D, and F, or hybrids thereof (e.g., class A/B, Class E/F, etc.). These different classes of power amplifiers differ with respect to method of operation (linear mode, switched mode), efficiency, output impedance and power output capability. For applications in which highly efficient power amplifier designs are required, switched mode Class D, E or F power amplifiers are typically implemented, as opposed to linear mode class A, B or AB power amplifier designs. Indeed, with switched-mode power amplifier designs, high efficiency is obtained by operating the active device(s) as a switch rather than a controlled current source, wherein various tuning methods can be implemented to minimize or eliminate overlap between the voltage and current waveforms across the switching devices to thereby reducing power dissipation (referred to as “zero voltage switching”). High efficiency power amplifiers are typically used in applications such as power supply converter and power supply regulator circuits, for example, where zero voltage switching is needed to reduce power consumption.
For high-efficiency switched-mode power amplifiers, the highest achievable frequency of operation is limited by various factors such as the type of switching devices implemented, for example. Power amplifiers providing Class E operation at high frequencies typically use MESFET, HEMT, or MOSFET switching devices. For example, Class E amplifiers operating at 10 GHz are known to use GaAs MESFET switching devices, but Class E amplifiers frequencies greater than 10 GHz are not known to exist. Indeed, Class E amplifiers using bipolar transistors, for example, are generally restricted to lower operating frequencies and bipolar power amplifiers operating at millimeter wave frequencies are typically designed using Class A or Class A/B operating modes.